Switching power supplies may be called upon to supply power to a load over a wide range: relatively large amounts of power may be required by a load in a “normal” operating mode, whereas relatively very little power may be required by the load during a “standby” operating mode. For example, an LCD flat-panel television receiver may require 350 Watts of operating power in normal operation but may only draw a small amount of power (e.g., 1 or 2 Watts) in a standby mode (i.e., when it is “turned off”) in order, e.g., to keep remote-control circuitry active.
Power supplies that operate from AC utility lines (“off-line power supplies”) may be called upon to meet special regulatory and performance requirements. For example, if the power drawn by a device from the AC utility line exceeds a certain limit (e.g., 75 Watts), it may be a requirement that the power supply in the device operate in a power-factor-correcting mode; on the other hand, it may also be a requirement that the power supply draw less than a specified amount of power (e.g., 5 Watts) from the utility lines when the device is in a standby mode of operation, in order to minimize energy utilization.
Off-line switching power supplies with power factor correction are well known in the art. Such supplies typically comprise a rectifier, to convert the AC utility line voltage into a unipolar pulsating input voltage, and a DC-DC converter that comprises one or more switches that are controlled by a controller. The controller controls the switches, at a switching frequency that may be much higher than the frequency of the AC utility source, to convert the unipolar pulsating input voltage into one or more controlled DC voltage outputs while also performing power factor correction (e.g., by forcing the waveshape of the current drawn from the AC utility source to follow the waveshape of the voltage delivered by the AC utility source). In normal operation, when supplying relatively high levels of power, the relatively high frequency switching may take place essentially continuously and the power supply may exhibit relatively high conversion efficiency. Continuous operation at relatively low load power levels, however, may present difficulties: e.g., it may be difficult to achieve switch duty cycles that are sufficiently low to maintain voltages within regulation; switching losses in continuous operation may result in objectionable losses. Lowering the switching frequency may produce audible noise.
One way to modify operation of an off-line, power-factor-corrected, isolated, switching power supply, at low power, is described in a datasheet for a power supply controller entitled TEA1750 GreenChip III SMPS control IC, Rev. 1-6 Apr. 2007, published by NXP Semiconductor, Eindhoven, the Netherlands. The NXP controller operates in a first operating mode, in which power-factor-correction and output control are performed at a continuous high switching frequency, when the power delivered by the power supply is high; the controller operates the power supply in another operating mode, called a “burst mode,” when the power delivered by the power supply is low. In burst mode, the power supply alternates between periods during which high-frequency switching occurs and periods during which no switching occurs. The NXP controller varies the frequency of the bursts as a function of the load power and a value of a circuit capacitor; audible noise may be reduced by initiating a “soft-start” mode during each burst. Another version of standby mode control is described in Rehm et al, U.S. Pat. No. 6,434,030, Arrangement Having a Switched-Mode Power Supply and a Microprocessor, issued Aug. 13, 2002. In Rehm, an off-line, isolated, flyback switching power supply is controlled to operate at low power in a burst mode at a pre-defined burst repetition frequency (e.g., 100 Hz) that is programmed into and controlled by a microprocessor. Yet another technique is described in Leonardi, U.S. Pat. No. 4,937,728, Switch-Mode Power Supply with Burst Mode Standby Operation, issued Jun. 26, 1990. In Leonardi, a burst mode in an off-line, isolated, flyback switching power supply is synchronized to the frequency of the utility line, the length of the each burst being essentially fixed to correspond to one-quarter of each AC utility line cycle.